The invention relates to magnetic resonance (MR) imaging, and more particularly relates to MR imaging of structures having varying magnetic susceptabilities. In its most immediate sense, the invention relates to MR imaging of veins in a living subject such as a human patient.
It has long been known that the magnetic susceptibility of blood varies with the state of oxygenation of the blood, and that de-oxygenated venous blood has a different magnetic susceptibility than oxygenated arterial blood. It has accordingly long been known that venous blood produces a smaller MR signal than arterial blood, and that this difference can be decreased by using an MR pulse sequence having a longer echo time. However, this phenomenon has not been utilized in MR imaging.
Furthermore, even though this relationship between magnetic susceptibility and blood oxygenation has long been known, it has not been used to evaluate the oxygenation of blood, nor to evaluate the total blood volume in a particular region of the body.
One aspect of the invention proceeds from a realization that by optimizing the echo time in an MR pulse sequence, it is possible to distinguish structures having different magnetic susceptabilities, which structures would otherwise look the same (or similar) in an MR image. More specifically, by using an echo time that is sufficiently long to cause different partial volume signal cancellations from the structures as compared with the background material in which they are embedded, it is possible to distinguish between them.
Another aspect of the invention proceeds from a realization that such optimization, when used in an MR pulse sequence of a velocity-compensated type, makes it possible for an MR study to distinguish veins from arteries and from surrounding tissue, and to quantify the oxygenation and volume of blood in the region of interest.
Hence, in accordance with one aspect of the invention, there is disclosed a method of MR imaging a region of interest containing at least two structures having different magnetic susceptibilities and a background material in which said structures are embedded. In accordance with the invention, an MR study of the region of interest is conducted using a MR pulse sequence in which the echo time is selected to cause different partial volume signal cancellations from the structures as compared with the background material, and MR data are acquired from gradient echoes produced by the MR study. From these data, each of the structures can be distinguished from the other, and from the background material.
In accordance with another aspect of the invention, which is specifically directed to an application in which veins are to be distinguished from tissue in which they are embedded, an MR study is conducted using a velocity-compensated MR pulse sequence. The echo time of the MR pulse sequence is selected to cause a partial volume signal cancellation from veins in the region of interest as compared with the background tissue in the region of interest. Then, MR data are acquired from gradient echoes produced by the MR study. From these data, the veins can be visualized.
In accordance with still another aspect of the invention, which is specifically directed to an application in which tissue-embedded veins and arteries are to be distinguished from each other and from the tissue in which they are embedded, an MR study is conducted using a velocity-compensated MR pulse sequence. The echo time of the MR pulse sequence is selected to cause a partial volume signal cancellation from veins in the region of interest as compared with arteries and background tissue in the region of interest. Then, MR data are acquired from gradient echoes produced by the MR study. From these MR data, an amplitude image and a phase image of the region of interest are formed, and the amplitude image is filtered by operating upon it with the phase image, thereby producing a filtered image.
The phase image is so constructed that it operates differently upon amplitude image data relating to veins and amplitude image data relating to arteries and background tissue. In this manner, veins and arteries are made more visually distinguishable.
In one advantageous embodiment, a minimum intensity projection or a maximum intensity projection or both are formed from the filtered image. The minimum intensity projection highlights veins; the maximum intensity projection highlights arteries.
In accordance with yet another aspect of the invention, oxygen saturation of blood and venous blood volume in a region of interest can be measured by conducting an MR study of the region of interest using a velocity-compensated multi-echo MR pulse sequence, acquiring MR data from gradient echoes produced by the MR study, and determining the oxygen saturation or venous blood volume from such data.